As methods for biological assessment (bioassay) of chemical substances, especially as methods for evaluation of harmfulness of chemical substances, methods based on determination of the viable cell count using cultured cells such as mammalian cells have been established as alternatives to animal experiments using mice, rats, or the like. In methods commonly employed in the previous studies, the viable cell count is determined by measurement of the absorbance, and a decrease in the number of the cells is measured. For example, the MTT method, which uses a reductive coloring reagent and mitochondrial dehydrogenase activity in live cells (Non-patent Document 1); and the XTT method (Non-patent Document 2) and the WST method (Patent Document 1, Patent Document 2, Non-patent Document 3), which are colorimetric methods based on measurement of enzymatic activity that causes reduction into MTT-like dyes; are widely used at present from the viewpoint of safety of measurement, high reproducibility, and the like. In particular, in the WST method; water-soluble formazan is produced after the addition of the reagent, so that the method allows sensitive and simple determination of the viable cell count, and stability after the preparation of the aqueous solution is high. However, these methods have a drawback in that a long time is required for allowing the cultured cells to respond to harmfulness of the chemical substance, finally showing a phenotype causing cell death.